Trench MOSFETs are well known. Referring to FIG. 1, a conventional trench MOSFET includes a plurality of laterally spaced trenches 6, base region 3, and a plurality of source regions 4 formed in epitaxial layer 20 which is disposed over substrate 1. Base region 3 has a conductivity opposite to that of source regions 4, substrate 1 and drift region 2 (the portion of epitaxial layer 20 between the bottom of trenches 6 and substrate 1), and is disposed between source regions 4 and drift region 2. Typically, epitaxial layer 2 is lightly doped relative to substrate 1.
The controllable gate structure of a typical trench MOSFET includes polysilicon gate electrodes 5 which are disposed inside trenches 6 and electrically isolated from source regions 4, base region 3, and drift region 2 by a layer of gate oxide 15. In the example shown by FIG. 1, substrate 1, drift region 2, and source regions 4 include N type dopants, while base region 3 includes P type dopants. The polarity of these regions is noncritical and may be reversed.
A typical trench MOSFET also includes high conductivity contact regions 8 of the same conductivity as base region 3, but of higher concentration of dopants, formed in epitaxial layer 20 between source regions 4. High conductivity contact regions 8 and source regions 4 are in ohmic contact with source contact 9 which is formed of a suitable contact metal such as Al or AlSi. It should be noted that source contact 9 is insulated from gate electrodes 5 by insulation interlayers 7 which may be formed from an oxide. A conventional trench MOSFET also includes drain contact 10 which is in ohmic contact with substrate 1. A trench MOSFET operates by applying an appropriate voltage to gate electrode 5 in order to create invertible channels in base region 3 adjacent the side walls of trenches 6 to electrically connect source regions 4 to drift region 2, thereby electrically connecting source contact 9 to drain contact 10.
Referring to FIG. 2, when the trench MOSFET of FIG. 1 is reverse biased electric field crowding is observed near the bottom corners of the trenches as schematically shown. The crowding of the field lines create high electric fields that can cause breakdown to occur near the gate oxide interface resulting in hot carrier injection into the gate oxide, breakdown voltage walkout and an unreliable gate oxide.
Under reverse biased conditions, the p-n junction between drift region 2 and base region 3 depletes back into base region 3. If the depletion layer reaches high conductivity regions 8 defects such as stacking faults provide a path for the leakage current Idss. To avoid this result base region 3 may be made sufficiently thick or its resistivity manipulated to improve the breakdown voltage rating of the device. This solution may not be ideal in all cases, however, because increasing the thickness and resistivity of base region usually leads to the increase of ON resistance, Rdson, which is undesirable.
Another known trench MOSFET is shown in FIG. 3. In this trench MOSFET like numerals identify the same features as those described earlier with respect to the trench MOSFET of FIG. 1. The trench MOSFET shown in FIG. 3 includes a deep high conductivity region 11 which is of the same conductivity type as, and extends below, base region 3 into drift region 2.
Deep high conductivity region 11 is formed through a high dose, implant and drive at the initial stages of the MOSFET fabrication process. FIG. 4 shows a plot of the doping concentration along line 4—4 in FIG. 3. The function of deep high conductivity region 11 is to move the location of breakdown away from the bottom corners of trenches 6 to the bottom of deep high conductivity region 11.
Deep high conductivity region 11 is usually as wide as it is deep which is disadvantageous in that it severely limits the cell pitch that is achievable (smaller cell pitches typically result in lower on-resistance). Also, the breakdown voltage rating is not improved by the inclusion of deep high conductivity region 11, and may be in fact reduced despite the fact the location of the breakdown is moved from the corners of the trenches to the bottom of the deep high conductivity region 11.